Abstract: Aimed at the problem of ammonium bisulfate (ABS) deactivated SCR catalyst at low load operation, the characteristics of ABS deactivation and activity recovery are studied in the simulated flue gas environment in laboratory. It indicates that the ABS liquidation-vaporization under capillary condensation is a typical invertible process and the process is positively affected by the concentration of NH₃ and SO₃ in flue gas. In addition, the empirical formula used to evaluate the MOT under the actual operating condition is proposed. Based on the case analysis of the SCR operating at low load, the principle and method to use the experience formula MOT is given to guide the selection of the best plan for SCR operating at low load and the activity recovery of the ABS deactivated catalyst.